Abstract: A surgical instrument 20 for machining a precise cavity in a patient's bone for receiving a prosthesis 13 having a conical portion 6 and a triangular portion 8 is provided. The instrument 20 includes a first member 26 which has (i) a conical portion 44 which defines a longitudinal axis 40 and (ii) a guide path 46, 76 for receiving a movable second member 24. The movable second member 24 carries a triangle cutter 22. The movable second member 24 is moved along the longitudinal axis 40 to cut bone until an appropriate index 30 on the movable second member 24 is aligned with a reference surface 28 on the first member 26. This alignment indicates that the triangular cavity formed by cutter 22 is of the appropriate size for the triangular portion 8 of the prosthesis 13 which is to be implanted.